The advantage is that you can reuse the same CSS across many HTML pages. Have one copy of the CSS file, and include it in all of the pages. Then, if you want to change, say, the background color of all of the HTML pages, change the one CSS file. EveryHTML page that refers to the CSS looks different. W00f!

There’s not much new in the CSS rules themselves. Just the usual fonting and coloring. The only new things are on lines 12:

input {

and 16:

button {

They style all of the <input> and <button> tags on the page.

Notice how background-color works. It sets just the background color of the input fields and buttons.